I'm sorry to say that it's not a feature available in the Chilkat API. For now, the Chilkat API is focused on cross-platform, cross-programming language compatibility. Using streams tends to be specific to a single programming language. Chilkat may address this in the future, but it's not possible for now.

PS> An easy way to answer this question for yourself would be to examine the online reference documentation. If there are no methods for loading an attachment from a stream, then (of course) the feature is not present..